tools/memory-model/Documentation: Fix typo, smb->smp

The tools/memory-model/Documentation/explanation.txt file says
"For each other CPU C', smb_wmb() forces all po-earlier stores"
This commit therefore replaces the "smb_wmb()" with "smp_wmb()".

Signed-off-by: Yauheni Kaliuta <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Acked-by: Alan Stern <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

tools/memory-model/Documentation/explanation.txt

···
 Second, some types of fence affect the way the memory subsystem
 propagates stores.  When a fence instruction is executed on CPU C:
 
-	For each other CPU C', smb_wmb() forces all po-earlier stores
+	For each other CPU C', smp_wmb() forces all po-earlier stores
 	on C to propagate to C' before any po-later stores do.
 
 	For each other CPU C', any store which propagates to C before